Traditional methods of extracting zinc from sulphides involve roasting the concentrate to remove sulphur followed by reduction during which some or all of the zinc is removed as a fume. The zinc is then recovered from the fume by condensation.
Similar processes are used for the recovery of other relatively volatile metals from sulphides. Although the invention will be herein described primarily with reference to the recovery of zinc it will be understood to be applicable to other metals including tin, lead, magnesium, cadmium, manganese or the like which may be produced by pyrometallurgical means involving fuming. The term "fume" as herein used refers primarily to a gas or vapour but includes a gas or vapour with entrained liquid and/or particulate matter.
Zinc blast furnaces, for example, generally use a lead splash condenser (e.g. such as described in British Patent 572961) to capture zinc vapour as zinc liquid and lower the activity of the condensed product. This approach involves relatively large capital investment and has high maintenance costs.
Another form of metal vapour capture currently in use relates to removal of zinc as fume from slag from lead production facilities, with subsequent oxidation. In that process, zinc vapour is fumed from the surface of a molten slag bath and is allowed to travel upwardly until it reaches a location well clear of turbulence associated with the bath surface. At that location an oxygen-containing gas such as air is admitted, and both temperature and oxygen potential are raised to the point where the zinc vapour is rapidly converted to zinc oxide. The zinc oxide has a very much lower vapour pressure than zinc metal and as a result precipitates from the gas stream as a fine powder. The zinc oxide is suitable for further purification by electrochemical methods, but must first be captured in suitable dust extraction equipment. This very fine dust is difficult to handle, and its capture once again requires large capital investment.
It has been found that by appropriate adjustment of temperature and oxygen potential in a furnace it is possible to achieve commercially significant direct separation of metal as a vapour from sulphide containing materials. However, in this case the furnace offgas contains sulphur dioxide together with the metal vapour and the sulphur dioxide reacts with molten metal in the splash condenser. Accordingly, it has hitherto been considered that the vapour would need to be oxidized prior to recovery.